1. Technical Field
The present invention relates to a light emitting device, and more particularly, to a light emitting device having a plurality of luminous elements emitting different colors respectively.
2. Discussion of the Related Art
In recent years, an electroluminescent element (hereinafter, referred to as “luminous element”) attracts attention of many researchers. In particular, efforts to employ such a luminous element to an image display device or an illuminating device have been actively made.
Generally, since a material constituting the luminous element has a poor resistance property to the exterior environment, it is necessary to appropriately protect (seal) the element from the exterior environment. Conventionally, as a method of sealing the element to be shut from the exterior environment, there are a method for bonding a sealed substrate using a sealing resin, a method for covering the element with a protecting layer, or combining the above two methods.
The latter one of the above methods, that is, the method of covering the luminous element with a protecting layer has characteristics of realizing a long life of the luminous element since an inorganic based material having a good blocking property to moisture or air may be used.
However, in the case that the protecting layer covering the luminous element is thin in a layer thickness, a problem which blocking to moisture or oxygen is not sufficient may be raised. On the contrary, in the case that the protecting layer is thick in the layer thickness, a problem may be raised, in which fabrication tact is deteriorated because it takes a predetermined time or more to form the film. Further, in the case that the layer thickness of the protecting layer is thick, peeling-off due to a film stress may be generated. Thus, the protecting layer is normally formed with a uniform thickness in a predetermined range of the layer thickness (for example, 300 nm˜3 μm) by sufficiently blocking moisture or oxygen in order to prevent deterioration of the fabrication tact and prevent the peeling-off due to the film stress.
Further, a conventional technique of sealing the luminous element using the protecting layer is disclosed in, for example, Japanese Patent Publication No. 2002-231443 and Japanese Patent Publication No. 2002-252082. For example, JP 2002-231443 includes sealing by a protecting layer using a material having a refractivity in a range of the atmosphere through 3.5. Further, JP 2002-252082 includes forming an anti-reflective film on the surface of a sealing film in contact with the atmosphere.
Materials constituting the luminous element may be divided into two kinds according to their optical properties, that is, a transparent material forming a luminous layer, a charge transport layer, a charge injection layer, a transparent electrode and a protecting layer, and showing a relatively high refractivity in the wavelength range of visible rays, and a metal material (opaque material) forming an electrode or an electron injection layer, and showing a value that a real part of a complex refractivity is lower than an imaginary part thereof in the wavelength range of visible rays. A large light reflection is generated at the interface surface between the former (transparent material) and the latter (opaque material) since there exists a large refractivity difference at the interface surface therebetween. That is, the latter functions as an reflective layer with respect to the former. Further, a large light reflection is also generated on the surface of the protecting layer in contact with the exterior out of the element. Thus, light interference occurs between a plurality of reflection surfaces, and an efficiency to draw the light generated from a luminous layer toward the exterior out of the element (hereinafter, referred to as “luminous efficiency”) is varied in accordance with thickness difference of each layer.
In the case of forming a color image display using a Top Emission type light emitting element drawing light from a side opposite to a substrate, an opaque anode, an organic layer (including a functional layer such as a charge (hole/electron) injection layer, a charge (hole/electron) transport layer, and the like), a transparent or translucent cathode, and a protecting layer are sequentially formed on the substrate. Further, a metal material is mostly used for the translucent cathode, and if a transparent cathode is used, a metal material is typically used for an electron injection layer. Alternatively, an opaque cathode, an organic layer, a transparent or translucent anode, and a protecting layer may be sequentially formed on the substrate.
Recently, a Dual Emission type luminous element drawing light from both a substrate side and a side opposite to the substrate is used, and the Dual Emission type luminous element is composed of a transparent or translucent anode, an organic layer, a transparent or translucent cathode, and a protecting layer, which are sequentially formed on the substrate. Alternatively, the Dual Emission type luminous element may be composed of a transparent or translucent cathode, an organic layer, a transparent or translucent anode, and a protecting layer, which are sequentially formed on the substrate.
In the luminous element structured as described above, there particularly exists an interface surface having a large light reflection. For example, in the case of using a transparent cathode in the Top Emission type,                (a) upper surface of the protecting layer        (b) upper surface of the electron injection layer        (c) lower surface of the electron injection layer        (d) upper surface of the anode may be interface surfaces having a large light reflection.        
Further, in the case of using a translucent cathode in the Top Emission type,                (a) upper surface of the protecting layer        (b) upper surface of the cathode        (c) lower surface of the cathode        (d) upper surface of the anode may be interface surfaces having a large light reflection, respectively.        
Further, for example, in the case of using a transparent anode and a transparent cathode, and employing a metal material for an electron injection layer in the Dual Emission type,                (a) upper surface of the protecting layer        (b) upper surface of the electron injection layer        (c) lower surface of the electron injection layer        (d) lower surface of the anode may be interface surfaces having a large light reflection, respectively.        
Further, for example, in the case of using a transparent anode and a translucent cathode in the Dual Emission type,                (a) upper surface of the protecting layer        (b) upper surface of the cathode        (c) lower surface of the cathode        (d) lower surface of the anode may be interface surfaces having a large light reflection, respectively.        
In the interface surfaces listed as above, by adjusting a layer thickness of the luminous layer to a different layer thickness in accordance with a luminous wavelength of each luminous element in common with the four cases, a degree of light interference of each luminous element occurring in the intervals of (b)˜(c), (c)˜(d), and (b)˜(d) can be increased.
However, the layer thickness of the protecting layer cannot be adjusted in each luminous element to correspond to the degree of the light interference of the intervals of (a)˜(b), (a)˜(c), and (a)˜(d) in common with the four cases because the layer thickness of the protecting layer is uniform in each luminous element by the reason described as above. Therefore, the conventional luminous element has problems that a wavelength maximized in a luminous efficiency is significantly different from a peak wavelength of the light generated from the luminous layer, and as a result, a luminous intensity is reduced, and a color purity is deteriorated and a color reproducibility is deteriorated.
Further, the conventional luminous element has problems that a layer thickness deviation exists inside the layer thickness of the protecting layer, and if an average layer thickness of the protecting layer is beyond the layer thickness value showing a maximized luminous efficiency, a large luminous intensity deviation or large color deviation may be generated even with a uniform layer thickness deviation.